1. Field of the Invention
The present invention relates to a time-limiting protection and control circuit, and more particularly, to a time-limiting protection and control circuit for protecting an element by limiting a pulse width of an input pulse.
2. Description of the Prior Art
A specification of a conventional electronic product having light emitting diodes is usually defined in detail for introducing properties of the light emitting diodes well. Please refer to FIG. 1, which is a diagram of a curve relating to a forward current and a relative luminosity of a light emitting diode of an electronic product having a flashlight. As shown in FIG. 1, when an ambient temperature of the electronic product is 25° C., the curve relating to the forward current and the relative luminosity gradually rises, so as determine a rated continuous forward current, which is about 30 mA as shown in FIG. 1, of the light emitting diode, and for determining a maximal current, which is about 100 mA as shown in FIG. 1, of reaching a maximal luminosity. The rated continuous forward currents and the maximal currents of different light emitting diodes also differ. Take the light emitting diode described in FIG. 1 as an example, when the light emitting diode continuously luminesces in an intermediate degree for lighting up an electric light or a flashlight of a camera, the light emitting diode is biased with the rated continuous forward current or a smaller current instead of being biased with the maximal current. When the light emitting diode luminesces in a high degree, such as for lighting up a flashlight of a camera, the light emitting diode is biased with the maximal current, which is 100 mA as exemplified in FIG. 1, for reaching a maximal luminance. Please refer to FIG. 2, which is a diagram illustrating a relation between a duty ratio and an allowable forward current of the light emitting diode described in FIG. 1 and biased with the maximal current, for describing properties of a light emitting diode of a flashlight of a camera. As shown in FIG. 2, when the duty ratio of the light emitting diode is below 10%, the light emitting diode may be biased under the maximal current, which is 100 mA as exemplified above. However, when the duty ratio of the light emitting diode is above 10%, with the increase of the duty ratio and the ambient temperature of the light emitting diode, the allowable forward current for biasing the light emitting diode decreases significantly. Please refer to FIG. 3, which is a diagram of an ambient temperature versus an allowable forward current of the light emitting diode described in FIG. 1 while the light emitting diode operates continuously, for describing operating properties of the light emitting diode utilized for an electric light. As shown in FIG. 3, when the ambient temperature of the light emitting diode is below 25° C., a maximal allowable forward current of the light emitting diode is 30 mA. However, when the ambient temperature of the light emitting diode is above 25° C., the maximal allowable forward current of the light emitting diode decreases continuously. As a constant-current driving integrated circuit of an electronic device continuously provides a forward current for the light emitting diode, the ambient temperature of the light emitting diode increases accordingly. When the ambient temperature of the light emitting diode is above 85° C., as shown in FIG. 3, the light emitting diode burns down so that the electronic device malfunctions.
Please refer to FIG. 4 and FIG. 5. FIG. 4 is a diagram of a control integrated circuit having single function. The control integrated circuit shown in FIG. 4 may be utilized for a flashlight of a camera or an electric light of a cell phone. Through single current, the control integrated circuit may be utilized for managing a forward current of a light emitting diode, which is designed to be operated continuously or is required to manipulate a length of an operation time with a duty ratio. FIG. 5 is a diagram of a control integrated circuit having two functions. The control integrated circuit shown in FIG. 5 has two enabling terminals for simultaneously manipulating a duty ratio of a light emitting diode of a flashlight of a camera and a switch-on function of an electric light of a cell phone. Therefore, the control integrated circuit provides different currents at the same time for both the light emitting diodes biased for different usages. Both the control integrated circuits shown in FIG. 4 and FIG. 5 determine respective forward currents by enabling. However, when a corresponding microprocessor or a corresponding central processing of a control integrated circuit malfunctions or has certain executing errors so that an enabling signal of the control integrated circuit is continuously enabled, light emitting diodes of the control integrated circuit would be easily burnt down, as described in FIG. 3. Moreover, conventional mechanisms including an over current protection (OCP) and an over temperature protection (OTP) utilized for such control integrated circuits are not able to provide in-time and efficient protection for light emitting diodes.